This invention relates generally to assembly of patterned crystalline arrays of colloidal particles, and more specifically to using electromagnetic radiation to electrohydrodynamically assemble patterned crystalline arrays of colloidal particles.
The arrangement of colloidal particles in a crystalline array has a variety of potential applications. Immediate possibilities include, but are not limited to the production of a template for photonic band-gap material with two-dimensional channels, biological and chemical sensors (by functionalizing the surfaces of the colloidal particles), micro-chip reactors, and high-density data storage devices. Many additional uses may be realized in the future.
A variety of techniques have been attempted for the production of crystalline arrays of colloidal particles. However, only a few of these techniques have the ability to produce two-dimensional patterns. Those techniques that can produce two-dimensional patterns have other limitations.
A technique for assembling colloidal particles into a crystalline array by the application of an electric field was reported by Richetti et al., Journal of Physics Letters 45, 1137-1143 (1984). Although the details of the assembly mechanism are not fully understood, the process involves simple coulombic interactions which bring particles close to an electrode surface, together with lateral motion stemming from electrohydrodynamic or electroosmotic effects. Once the particles are close to the surface of the electrode, where they remain mobile, electrohydrodynamic or electrokinetic processes assemble them into crystalline arrays. Particles can be permanently attached to the electrode surface by increasing the attractive force between the particles and the electrode. When the attractive forces exceed the force of steric repulsion, entry into the xe2x80x9cprimary minimumxe2x80x9d creates a permanent bond.
Trau et al. (U.S. Pat. No. 5,855,753) teach a method for patterning a crystalline array of colloidal particles by altering the surface topography of the electrode in a electrolytic cell, and is incorporated herein by reference. The variations in surface topography cause nonuniform current density that affects the motion of particles near the electrode surface. Charged colloidal particles have a tendency to migrate toward areas of greater current density and increased current density can also cause the particles to assemble in crystalline arrays and become fixed to the electrode. However, this process has the disadvantage of added time and expense for lithography and etching processes. Also dimensional errors are introduced in the crystalline array due to lithography and etching process tolerances. Nor can this method be used for forming patterns of colloidal particles comprising different material or having different funtionalization.
G. M. Whitesides et al. (Advanced Materials 10 (13) 1045-1048 (1998) teach an alternative method for assembling patterned colloidal crystals. The presence of capillary forces in a microcontact printing mold draws a suspension of colloidal particles into small channels above the substrate, and evaporation of solvents allows the colloidal particles to self-assemble into colloidal crystals. This method eliminates the need to etch or otherwise alter the substrate. However, the reliance on capillary flow restricts this method to patterns consisting of interconnecting areas. A continuous channel for fluid flow must connect every part of the pattern to the edge.
A need still exists for a method for assembling patterned crystalline arrays of colloidal particles and affixing them to an electrode without physical modification of the electrode surface or the colloidal crystal and wherein the pattern of colloidal crystals does not have to be interconnected. It is an object of the present invention to provide a method for assembling patterned colloidal crystals using selective illumination of an optically-sensitive electrode with electromagnetic radiation. It is a further object of the present invention to provide a flexible process for forming patterned colloidal crystals.
To achieve these and other objectives, and in view of its purposes, the present invention provides a method for electrohydrodynamically assembling colloidal particles into a patterned crystalline assembly and permanently affixing the crystalline assembly to an electrode by selectively illuminating an electrode consisting of optically-sensitive semiconducting material with electromagnetic radiation while using the electrode to apply an electric field to the colloidal particles.
In one embodiment of the present invention, a method is provided for forming a patterned crystalline assembly of colloidal particles by illuminating selected areas of a semiconducting electrode with electromagnetic radiation while using the electrode to apply an electric field to the colloidal particles. The intensity of the electric field or the electromagnetic radiation or both can be modulated over time to enhance migration and assembly of the colloidal particles and to permanently affix the crystalline assembly of colloidal particles to the electrode.
It should be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.